I am using version 2019.1.215 and I am unable to import anything to fix the missing ScatterSeries classes.
SeriesGroup scatterSeriesGroup = chart.Chart.SeriesGroups.First() as ScatterSeriesGroup;
ScatterSeries scatterSeries = scatterSeriesGroup.Series.First() as ScatterSeries;
scatterSeries.Marker = new Marker();
scatterSeries.Marker.Symbol = MarkerStyle.Plus;
scatterSeries.Marker.Size = 22;
scatterSeries.Outline.Fill = new SolidFill(Colors.Red);